The present invention is directed to a sealing structure useful for forming a seal around chucks, particularly electrostatic chucks, which are used for holding substrates.
In integrated circuit manufacture, chucks are used to hold semiconductor substrates during processing to prevent movement or misalignment of the substrate. Electrostatic chucks, that use electrostatic attraction forces to hold a substrate, have several advantages over mechanical and vacuum chucks, including reduction of stress-related cracks caused by mechanical clamps; utilization of a larger portion of the substrate surface; reduced formation of corroded particles that deposit on the substrate; and allowing use of the chuck in low pressure processes. A typical electrostatic chuck includes an electrically conductive electrode with an electrical insulator thereon. A voltage source electrically biases the substrate with respect to the electrode. The insulator prevents the flow of electrons therethrough, causing opposing electrostatic charge to accumulate in the substrate and in the electrode, thereby generating an electrostatic force that attracts and holds the substrate onto the chuck. Electrostatic chucks are generally described in, for example U.S. patent application Ser. Nos. 08/278,787 by Cameron, et al.; 08/276,735 by Shamouilian, et al.; and 08/189,562, by Shamouilian, et al.--all of which are incorporated herein by reference.
One problem with conventional chucks is their limited lifetime in erosive process environments. A typical electrostatic chuck comprises a metal electrode covered by a thin polymeric insulator layer. The thin polymeric layer maximizes electrostatic attractive forces between the substrate and the electrode. However, when the insulator comprises a polymer such as a polyimide, the polymer insulator can be rapidly eroded by the erosive process environment, limiting the useful lifetime of the chuck. Erosion of polymer insulators can be particularly severe in oxygen or halogen containing gases and plasmas, which are used for a variety of tasks, such as for example, etching of substrates, deposition of material on substrates, and cleaning of process chambers and substrates. While a large portion of the polymer insulator is covered by the substrate held on the chuck and thereby protected from the erosive environment, the periphery of the insulator is exposed to, and eroded by the erosive gaseous environment. The exposed polymeric insulator can erode in a few hours (typically 2 to 3 hours of exposure to a oxygen plasma environment), and erosion through even a single point on the insulator can expose the electrode causing short-circuiting and failure of the chuck. Failure of the chuck during processing can damage the substrate and reduce yields of the valuable integrated circuit chips formed on the substrate. Also, the polymeric byproducts formed during erosion of the polymer often deposit on the chuck and walls of the process chamber to form hard polymeric deposits which are difficult to clean. Thus it is desirable to reduce the erosion of exposed portions of the chuck, particularly the polymer insulator layers on the chuck.
Several techniques have been developed to reduce erosion of the exposed portions of the chuck. In one solution, described in commonly assigned U.S. patent application Ser. No. 08/410,449, filed on Mar. 24, 1995, which is incorporated herein by reference, a flow of inert or non-reactive "masking" gas is directed onto the peripheral insulator portions of the chuck. The masking gas protects peripheral insulator from the corrosive effect of the surrounding gaseous environment. However, the concentration of masking gas can change the concentration of reactive process gas species adjacent to the edge of the substrate thereby altering the rate of processing, i.e., etching or deposition, at the peripheral edge of the substrate. Also, when the pressure of process gas in the chamber is more than one-tenth of the masking gas pressure, upstream diffusion of masking gas due to concentration gradients reduces the effectiveness of the masking gas. Thus, the masking gas method is undesirable in certain substrate fabrication processes.
Another erosion reducing method uses a barrier ring that extends continuously around the peripheral edge of the insulator and forms a seals against the edge of the substrate. A suitable barrier ring structure, disclosed in U.S. patent application Ser. No. 08/439,010, filed on May 11, 1995, which is incorporated herein by reference, comprises an elastomeric ring having a base resting on a support below the substrate, and an arm extending from the base. When an electrostatic attractive force is applied on the substrate, the substrate moves downward and presses against the upper portion of the arm of the barrier ring to form a seal that reduces exposure of the insulator of the chuck to the corrosive process gas. However, the barrier ring is compressed against the substrate during the entire process cycle and cannot be released from the chuck without terminating the voltage applied to the electrode of the chuck. Terminating the voltage to the chuck electrode releases the substrate from the electrostatic attractive force, which may cause the substrate move during processing. Movement of the substrate during processing is undesirable, particularly for processes where it is necessary for the substrate to be aligned in a particular orientation on the chuck.
The barrier ring can also have problems in highly corrosive process environments where the portion of the barrier ring contacting the substrate can stick to the substrate (due to corrosion in the corrosive process environments) making it difficult to remove the substrate after processing is completed. A slow release or sticking substrate can damage the substrate when a robotic lift arm attempts to lift the substrate off the chuck. Compensating for occasional sticking of the substrate by slowing the speed of the robotic arm, increases substrate processing time and reduces process throughput. Also, sticking portions of the barrier ring can leave behind undesirable contaminants on the substrate that further reduce substrate yields.
Another problem with conventional barrier ring structures arise from the difficulty in fabricating a barrier ring that has consistent and tight tolerance height dimensions (which are needed to apply uniform pressure along the entire peripheral edge of the substrate), particularly when the barrier ring is fabricated from a soft polymer. Conventional polymeric barrier seals typically have height variations that vary by over 250 microns (10 mils). These height variations result in gaps between the barrier ring and the substrate that allow ingress of corrosive gases that react with and erode the insulator of the chuck. To reduce such gaps, the arm of the barrier ring is fabricated with a slightly larger height than necessary to allow compression of the arm by the edge of the overlying substrate when the chuck electrode is powered and the substrate is electrostatically attracted to the chuck. The larger the "excess height" of the barrier ring, the more the variation in height across different portions of the ring that can be tolerated without adversely affecting the continuous seal around the substrate. However, the large height of the barrier ring combined with the variation in height due to manufacturing tolerance limitations creates an upward force on the substrate that opposes the electrostatic force with which the substrate is held on the chuck, increasing the possibility of movement of the substrate during processing. This is a particular problem for electrostatic chucks that provide reduced electrostatic attractive forces, such as bipolar chucks, and chucks required to operate at relatively low operating voltages (less than 2000 volts). If selected portions of the barrier are excessively high, the barrier ring can lift the entire substrate off the surface of such chucks. Also, the large height of the barrier seal further increases the problem of sticking against the substrate due to excessive compression of the seal.
Yet another problem with conventional electrostatic chucks arises from use of heat transfer fluid grooves on the chuck surface which are used to hold a heat transfer fluid for heating or cooling the substrate held on the chuck. Suitable configurations of such heat transfer fluid grooves are described in commonly assigned U.S. patent application Ser. No. 08/276,735 to Shamouilian, et al., which is incorporated herein by reference. When a substrate is held on the chuck, the substrate covers and seals the grooves to reduce leakage of heat transfer fluid held in the grooves. The substrate is typically cooled using a heat transfer gas held in the grooves to reduce overheating of the substrate, and to provide higher yields of integrated circuit chips, particularly at the peripheral edge of the substrate. However, the heat transfer gas often leaks out from below the peripheral edge of the substrate, resulting in overheating of the peripheral portion of the substrate. Also, the leaking gas can adversely effect the process gas composition at the edges of the substrate. Thus, it is desirable to have a sealing structure that reduces leakage of heat transfer gas from the edge of the substrate.
Thus, it is desirable to have a sealing structure to protect the chuck from erosion in the erosive process gas environment and to reduce leakage of heat transfer gas from below the substrate. It is also desirable for the sealing structure to be capable of being actuated or deactuated during processing of the substrate. It is further desirable for the sealing structure to allow rapid placement and removal of the substrate from the chuck without sticking to the substrate and forming contaminants on the substrate. It is also desirable for the structure to seal substantially the entire edge of the insulator without forming gaps through which erosive process gases can access and erode the chuck.